An antenna array comprises a number of individual antenna elements each radiating electromagnetic energy. The composite beam emitted from a fixed antenna array can be formed and steered by controlling the phase of the signal radiated from each element. Thus, phase control allows the antenna beam to be scanned over the area of interest without physical movement of the antenna. This feature is most important when an antenna must continually scan a large directional area in a short time, such as an airport surveillance radar system.
One mechanical technique for antenna steering is described and claimed in U.S. Pat. No. 4,241,352. This patent discloses a beam scanning microwave antenna system wherein the antenna array includes a plurality of antenna elements, wherein a single point signal feed network is provided with a scan network, to couple the single point feed network to the antenna array. The scan network comprises a plurality of coupling paths between the feed network and each antenna element. The coupling paths are formed, at least in part, by conducting elements disposed in concentric circular segments A rotating directional coupler couples the feed network to the concentric circular conducting segments for varying the lengths of the respective coupling paths in accordance with the rotation of the directional coupler. Varying the path lengths varies the phase relationship between the antenna elements in a scanning fashion across the plurality of antenna elements.
A limitation of this scanning device is that the signal path is through the rotating directional coupler. This requires precise electrical control of the coupler so that there are no discontinuities or other perturbations to influence the amplitude or phase of the signal finally transmitted from each antenna element. The rotating directional coupler directly controls the amplitude of the excitation of each respective antenna element and thus the resulting composite amplitude distribution across the antenna aperture. These rotating couplers will thus be very sensitive to physical spacing between the coupled transmission lines. Because the directional coupler must rotate, it is difficult to maintain a close tolerance between the coupled lines, and the desired amplitude distribution across the face of the antenna array may be accordingly degraded. Further, this antenna array must employ a directional coupler to the phasing line. Reduced directivity in the coupler produces a loss of radiation efficiency from the antenna array. Possible reflections from backward traveling waves could also reach the radiating elements and distort the amplitude and phase distribution.
An electrical phase shifter, such as the microwave phase shifter described and claimed in U.S. Pat. No. 3,005,168, can also be used with an antenna array to provide the scanning feature. A phase shifter is associated with each antenna element such that the phase shifted beam from each element constructively interferes with the beam from every other element to produce a composite beam radiating at an angle from a line normal to the aperture. By changing the phase shift provided by each phase shifter, the beam can be scanned across the antenna aperture.